The invention concerns a headbox for producing a multilayer or multi-ply paper web, and a method for producing such a paper web.
A multilayer headbox is known, e.g., from DE-OS 40 29 545 A1. It serves the production of a multilayer paper web, wherein the individual nozzle spaces are supplied with stock suspension independently of one another. Pivotable blades extend across the entire machine width and are arranged between the individual nozzle spaces of the multilayer headbox, thereby preventing a premature mixing of the various stock flows.
Also known are paper machines for producing multiply paper layers with the aid of several headboxes acting mutually independently, such as described, for example, in German Patent Disclosure DE 40 31 038 A1. This document describes a paper machine in which there are plies of paper produced on several jointly running wires, independently of one another, which in subsequent processing--that is, still before the press section of the paper machine--are superposed to create multi-ply paper.
An unsolved problem with these multilayer and multiply papers is adjusting the basis weight cross profile and the fiber orientation cross profile of the web. The basis weight of the paper web is the weight of the paper per square unit. Although the basis weight should be constant over the width of the paper web, in practice it generally is not. The deviations of the basis weight from a desired value may be shown in a curve or "profile." The fibers (of a length of about 1 mm or less) should generally be oriented in the machine direction. In many cases, however, there may be a deviation from this direction. A "profile" of the fiber orientation will be created in the cross direction of the paper web. Notably, with a three-layer headbox it is very difficult to influence the center layer. Problems regarding the fiber orientation of the outer layers arise at the same time due to diaphragms provided on the nozzle.
The problem underlying the invention is to present for the preparation of a multi-ply or multilayer stock web a multilayer headbox that makes it possible to influence, independently of one another, the cross profiles of fiber orientation and basis weight in the individual plies and, thus, to provide a paper web with improved flatness, improved basis weight profile along with economical production.